In radio communications, there are different ways to divide and access radio resources to effect a communication. In frequency division multiple access (FDMA), a radio channel is typically assigned to a narrow frequency bandwidth for the duration of a call. In time division multiple access (TDMA), radio resources are divided and accessed using a narrow frequency bandwidth and a time period or time slot. In Code Division Multiple Access (CDMA), all users may transmit over the same relatively wide bandwidth. The wide bandwidth allows for a high degree of channel coding. Users can therefore be distinguished at a receiver by an assigned pseudo-noise code sequence.
The capacity of a radio communications system may be increased by dividing the system's geographical coverage area into cells. Radio channel resources, e.g., radio frequencies, time slots, code sequences, etc., used in a first cell may be re-used in a second cell located a sufficient distance from the first cell. That distance should be sufficient so that interference between the areas does not substantially degrade quality and performance in either cell.
Thus, a limiting factor for channel re-use is interference, and in particular, co-channel interference caused by nearby radios using the same radio channel. However, increasing amounts of co-channel interference may be tolerated depending on the amount and sophistication of signal processing at the receiver, e.g., coding, diversity, interleaving, etc. The more co-channel interference receivers can tolerate, the more system operators can decrease the re-use distance, plan a “tighter” network, and thereby increase capacity. Network planning distributes the limited radio resources in a certain reuse pattern over the coverage area so that the quality throughout the area is adequate given the number of subscribers and types of services to be supported.
FIG. 1 illustrates, for an cellular system, an example cellular pattern with a 1-to-3 re-use factor where the available spectrum is divided into 3 separate frequency groups all of which are used within one base station site. A mobile station 10 communicates on frequency F2 with base station 12. As the mobile station 10 moves to another location in the network, a handover may be performed to another frequency in another sector, e.g., it may move to be served by base station 14 on frequency F1.
Re-use in TDMA systems involves time slots as well as frequencies. FIG. 2 illustrates a timeslot pattern on one frequency as specified for a GSM type of mobile radio communication system. A GSM cellular radio system is described here and below as an example of a cellular radio system. However, it is understood that the following description and the invention are not limited to GSM. In GSM, there are 8 timeslots (TN0–TN7) distributed on each carrier frequency. A user is allocated one (or more) of these timeslots for a communication. Each repeated cycle of all timeslots is a frame. The GSM cellular standard specifies a set of repetition patterns for frames related to frame numbering and repetition of certain control channel information. The GSM frame structure is described in the 3GPP TS 45.002, ver. 4.3.0 specification, the disclosure of which is incorporated here by reference.
A modulated signal transmitted during a timeslot on one frame is commonly referred to as a burst. A GSM burst structure specifies different parts of the burst to carry different types of information. With Gaussian Minimum Shift Keying (GMSK) modulation specified for GSM, each burst contains 114 coded data bits. The GSM burst structure is illustrated in FIG. 3 and is essentially symmetrical around a symbol field called a training sequence (TS) 38. This training sequence is a known symbol sequence used by the radio receiver to estimate the parameters for a model of the current radio channel. That model estimates how the current radio channel has distorted the burst as a result of transmission over the air. Two flag bits 36 surrounding the training sequence indicate the type of information included in the data fields, i.e., user data or system signaling information. The user data bits are separated into two different fields 34, and the burst begins and ends with a tail bit field 32 extending over 3 modulation symbols.
The training sequence 38 contains 26 modulation symbols corresponding to 26 bits, since GMSK is a binary modulation, and there are 8 different training sequences defined for GSM. The training sequences have very good autocorrelation properties meaning that a convolution of a training sequence X(n) with itself generates a high correlation result. However, convolving training sequence X(n) with a shifted version of that same sequence, X(n+d), d≠0, generates a zero correlation result, or close thereto. The good autocorrelation properties of the training sequences make for easy and accurate synchronization and radio channel estimations.
The burst structure illustrated in FIG. 3 is normally used for both uplink and downlink user communications in GSM. For example, full rate speech CODECs in GSM interleave 20 ms of coded speech, (i.e., a speech frame), on 8 consecutive bursts, each burst having eight time slots. Bit interleaving distributes consecutive speech codec output information, i.e., bits from the same frame, in an intelligent way on several bursts so that if one burst is lost in the transmission over the radio channel, it may still be possible to accurately decode the 20 ms speech with the help of the channel coding. Indeed, coding redundancy together with interleaving could permit proper detection of transmitted user information even over poor radio channel connections.
Enhanced radio transmission performance, whether by improved coding, improved transmission techniques, improved receiver techniques, etc., may permit a decrease in re-use distance in the cellular network. Decreased re-use distance, (sometimes referred to as “tight reuse”), permits increased network capacity. In tight re-use networks, occasional interference which only corrupts one or a few bursts is usually not a significant problem because of channel coding and interleaving. However, if there are significant and fairly consistent amounts of interference, especially co-channel interference from a close-by interferer operating on the same frequency, a larger number of bursts may be adversely effected to the point where they can not be satisfactorily compensated for using interleaving and channel coding. In this regard, co-channel interference is just one of a myriad of signal degrading phenomena that adversely affect mobile connections including small-scale fading (also called multipath, fast, or Rayleigh fading), large scale fading (also called log-normal fading or shadowing), path loss, and time dispersion. It is thus desirable to spread the effects of signal degradation in the system so that an individual connection experiences a varying signal degradation.
To spread interference or other factors that degrade the quality of the signal, frequency hopping may be used. FIG. 4 illustrates the frequency hopping concept in GSM using the GSM frame structure. Each GSM frame is transmitted on a different frequency. Bursts belonging to one connection “frequency hop” between each frame, i.e., consecutive bursts are transmitted on different frequencies. The frequency hopping algorithm in GSM allows for different types of hopping patterns including pseudo-random and cyclic. Users are separated both by different frequency hopping patterns and different offsets in the frequency hopping pattern. The GSM frequency hopping is more thoroughly described in the 3GPP TS 45.002 specification referred to above.
With frequency hopping, each burst in a connection experiences different interference levels from different channels and different cells. As a result, the likelihood that there will be a single strong interference source throughout a consecutive number of bursts is significantly decreased. Because a connection encounters different interfering sources, frequency hopping, especially pseudo-random frequency hopping, provides a kind of interference diversity. In this way, random or pseudo-random frequency hopping is particularly useful to average co-channel interference and spread interference over all users in the system. Frequency hopping, (especially cyclic frequency hopping), is also useful at providing frequency diversity to compensate for different types of fading of the radio signal including multi-path fading. For a particular frequency, if there is a fading dip in the exact position of the receiver, changing the frequency moves the fading dip, and the probability that a new fading dip will be located at the exact same position is low.
These interference averaging benefits associated with frequency hopping are particularly advantageous in a time-synchronized system. In a time-synchronized system, different base station sectors or cells at a single base station site transmit a burst at the same instant, i.e., a start of a burst occurs at the same time. If the system is time-synchronized, a user will be interfered by the same source throughout the whole burst. This makes it possible to avoid interference from close-by neighbor cells by allocating frequency hopping sequences in an intelligent way, usually referred to as mobile allocation index offset (MAIO) management.
In any cellular network trying to deal with co-channel interference and other signal degrading phenomena, even in a tight-reuse, frequency hopping network, it is useful to explore other ways to differentiate between user or other different transmissions. To this end, the present invention employs training sequences. Training sequences include a sequence of symbols known by both a transmitter and a receiver and are used by the receiver to determine how the current radio channel and interference is affecting transmitted symbols. By determining the difference between what symbols were transmitted and what were received, a channel model can be estimated and updated. That channel model can then be used to determine the values of transmitted symbols whose values are unknown to the receiver. Accordingly, transmission from a serving base station may be identified by correlating it to a certain training sequence during the training sequence period. Different training sequences may be purposefully selected for close-by cells using the same radio channels for communication. This purposeful training sequence selection/assignment to differentiate between cells using the same frequency requires a new dimension in the planning of a network; it requires training sequence planning.
Moreover, in a time-synchronised system with a tight frequency reuse, as described above, the training sequence planning will be complicated because the training sequence transmissions from different base stations will coincide/collide in some mobile stations. This is illustrated in FIG. 5. Serving base station 52 transmits a burst 58 from using the frequency F1 during the timeslot TN1. Neighboring base station 54 transmits a burst 59 from using the same frequency F1 during the same timeslot TN1. Mobile station 56 receives the two bursts 58 and 59 simultaneously. Despite this “collision,” if bursts 58 and 59 are transmitted using different training sequences, the mobile station can separate the two bursts.
In order to use training sequences to distinguish between bursts, it is important for those sequences to have good cross-correlation properties in addition to good auto-correlation properties. Good cross-correlation means that the convolution of two different training sequences, e.g., X(n) and Y(n), results in a small or zero value. Without good cross-correlation properties, it is difficult to separate two transmissions, e.g., two bursts received at the same time from base stations 52 and 54. Unfortunately, training sequences typically used in current cellular radio communications systems, e.g., training sequences defined for GSM, do not necessarily have consistent, good cross-correlation properties. Hence, correlation performance depends on the two specific training sequences being correlated. Although some training sequence pairs may have good cross-correlation properties, other pairs will have poor cross-correlation properties. The latter is a problem if such a pair is used in close-by cells to distinguish between user bursts. This means that training sequence planning becomes more complicated since close-by neighbor cells should use not only different training sequences, but also different training sequences with good cross-correlation properties.
It is an object of the invention to permit tight frequency-reuse while distributing co-channel interference and/or other signal degrading phenomena.
It is an object of the present invention to employ training sequences to distinguish between different user bursts without having to perform significant associated network planning.
It is an object of the invention to employ training sequences to distinguish between different user bursts without requiring all training sequences to have good cross-correlation properties.
The present invention overcomes the problems described above and meets these and other objects using training sequence hopping. During a first portion of a transmission, e.g., a first burst, a first training sequence is used. During a second portion of that transmission, e.g., a second burst, a second different training sequence is used. Different training sequences are used for different portions of the transmission according to a predetermined pattern and with a predetermined number of training sequences. The portion duration can extend for the length of a burst, for more than one burst, for a speech frame, or for some other time period.
For transmissions in either or both the uplink and downlink directions, different training sequences are used in a pseudo-random, cyclic, or other manner for different portions of a transmission. Information regarding a training sequence hopping pattern to be used for a certain transmission burst or other portion is communicated between appropriate base and mobile stations. For example, a training sequence hopping pattern may be parameterized and sent as a part of control signaling for a call setup procedure, or after a new channel allocation has taken place. In a receiving station, the signaled training sequence hopping pattern is used to select the appropriate training sequence to correlate with a received transmission portion.
A training sequence hopping pattern generator generates a training sequence hopping pattern for a certain transmission. The hopping pattern generated may be pseudo-random, cyclic, or some other pattern. In one example embodiment, a training sequence hopping pattern is determined using a parameter set including one or both of a number of training sequences in the hopping pattern and a current frame number. A hopping offset may also be employed.